This invention relates to synchronous motor systems and particularly to stepping motor systems.
Stepping motors and their operation are well known. In an article entitled "New Inductor Motor Has Low Speed, Self-Start and High Torque" by J. H. Staak in the June 1947 edition of Electrical Manufacturing, Vol. 39, No. 6, published by the Gage Publishing Co., the author describes an early stepping motor. In such a motor, a rotor composed of two axially displaced sets of permanently-magnetized radially-extending rotor teeth rotates about an axis common to a stator composed of inwardly extending stator poles each having stator teeth. One set of rotor teeth is peripherally displaced relative to the other set by 1/2 tooth pitch. The stator teeth extend axially across both sets of rotor teeth.
One of the advantages of a stepping motor is its ability to rotate in synchronism with an applied voltage. Another advantage is its ability to "step" or rotate the rotor through a predetermined stepping angle in response to an input voltage or current pulse. This is of special importance where the motor operates in response to digital data. In general, the stepping angle, or the angle of a step, through which a rotor responds to a single pulse is determined by the number of rotor teeth, i.e., the rotor tooth pitch, and by the number of poles. Another advantage of a stepping motor is that it is possible to provide for cogging during rotation or for smooth rotation depending upon the desired application, by selecting a proper stator tooth pitch relative to the rotor tooth pitch.
Stepping motors have a number of disadvantages. The stepping angle is a function of the number of rotor teeth. Thus, the smaller the desired stepping angle the greater the number of teeth required. Thus, motors requiring very fine resolution require rotors with small teeth. However, the machinery for manufacturing different types of rotors to satisfy different resolutions is expensive.
Moreover, the pulse generators which are used to drive large motors require power transistors with large current carrying capacities. The current carrying capacities of the transistors used limit the power which can be delivered to the motors. It is difficult to operate transistors in parallel to achieve high power drives, because variations in transistor characteristics may cause the transistors to carry currents unequally and eventually burn out the transistor carrying the highest current.
Furthermore, stepping motors tend to exhibit resonances. It is not known why such resonance exist. It has been suggested that such resonances occur as a result of the starting and stopping shocks during each step. Stepping motors also tend to vibrate.
As a further point, the stepping motor stepping angles, because they are a function of the number of rotor teeth, are usually invariable once the motor has been built.
An object of this invention is to improve stepping motors.
Another object of this invention is to alleviate the aforementioned problems.